Thermal energy, such as generated by high intensity focused ultrasound (acoustic waves with a frequency greater than about 20 KHz, and more typically between 50 KHz and 5 MHz), may be used to therapeutically treat internal tissue within a patient. For example, ultrasonic waves may be used to ablate tumors (e.g., breast tumors and uterine fibroids), thereby obviating the need for invasive surgery. For this purpose, a phased transducer array having transducer elements driven by electrical signals to produce ultrasonic energy can be placed external to the patient, but in close proximity to the target tissue mass to be ablated.
The transducer is geometrically shaped and positioned, such that the ultrasonic energy is focused at a “focal zone” corresponding to the target tissue mass within the patient. During the wave propagation across tissue, a portion of the ultrasound energy is absorbed, leading to increased temperature and eventually to cellular necrosis, preferably at the target tissue mass at the focal zone. The focal zone of the transducer can be rapidly displaced by independently adjusting the amplitude and phase of the electrical signal input to each of the transducer elements. The focal size and focal length of the ultrasound transducer will depend on the ultrasound frequency, the focal depth and the aperture size of the transducer. Because the size of target tissue mass is often greater than the size of the focal zone, the transducer may be sequentially focused and activated at a number of target sites within the target tissue mass to fully coagulate the target volume. The sequential “sonications” are used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
Image-guided focused ultrasound therapy systems offer the benefit of target visualization and localization. In particular, before a focused ultrasound treatment procedure is performed, a patient may be initially imaged to localize the mass and/or to plan a trajectory of the ultrasound beam. For example, using displayed images of the internal body region, a treatment boundary can be defined around the target tissue mass, and obstacle boundaries can be defined around tissue that should not be exposed to the ultrasound energy beam. The ultrasound transducer can then be operated based on these defined boundaries. During treatment, the patient can be continuously imaged to ensure that the target tissue mass is treated without damaging surrounding healthy tissue.
Magnetic Resonance Imaging (MRI) guidance offers the additional benefit of temperature mapping in vivo, which can be used to verify that a sufficient temperature is reached during each application of ultrasonic energy (i.e., sonication) to kill the target tissue mass or portion thereof. Temperature mapping can be accomplished by measuring the temperature change (rise) of the portion of the tissue mass being heated during each sonication using conventional MR imaging techniques coupled with image processing to extract the temperature from the MRI data. Thus, accurate temperature measurements allow verification of the proper location of the focal zone and computation of the accumulated thermal dose during treatment for prediction of tissue ablation.
Current image-guided focused ultrasound therapy systems, such as those based on MRI, assume that the acoustic transducer is in a predefined and known position relative to the target tissue mass to be treated. In this case, the respective coordinate systems defined by the imaging device, therapeutic device, and patient remain registered with each other. If unplanned movement of either the transducer or the patient is detected, however, one or both of these coordinate systems become mis-registered with the other coordinate systems. As such, the treatment process must be stopped and the ultrasound beam trajectory re-planned. This introduces significant inefficiencies in the treatment process and may generate significant delays. Some image-guided focused ultrasound therapy systems use mechanically aligned imaging and ultrasound transducer arrangements or use the same transducer to perform both imaging and therapy tasks. In these cases, the coordinates of the therapy system remain registered with the coordinates of the imaging system if the transducer moves. However, if the patient moves, the patient coordinate system in which the target tissue region is defined, will become mis-registered with the imaging and therapeutic coordinate systems.
Current generation of systems require that all three coordinate systems—patient, imaging, and therapeutic, be locked or be continually registered with respect to each other, so that treatment of the target tissue region without damaging surrounding healthy tissue can be assured and verified. Conventionally, this has been accomplished by mechanically aligning the imaging and therapy transducers, as briefly discussed above, and immobilizing the target tissue region. However, electronic phasing changes or electrical impedance changes within the control circuitry of the transducer may still generate misalignment between the imaging and therapy beams; that is, cause mis-registration between the imaging and therapeutic coordinate systems. In addition, immobilization of the target tissue region of the patient may not always be practically accomplished.
For example, after delivery of a thermal dose, e.g., an ultrasound sonication, a cooling period is required to avoid harmful and painful heat build up in healthy tissue adjacent the target tissue mass. This cooling period may be significantly longer than the thermal dosing period. Since a large number of sonications may be required in order to fully treat the target tissue mass, the overall time required can be quite significant. This means that the patient must remain motionless in the imaging device for a significant period of time, which can be very stressful. At the same time, it may be critical that the entire target tissue mass be ablated (such as, e.g., in the case of a malignant cancer tumor), and that no short cuts be taken during the procedure just in the name of patient comfort. Thus, the use of image-guided focused therapy systems are limited to treating tissue masses with small motion amplitudes or to those that are easily immobilized.
Recently, it has been suggested that the movement of a target tissue mass relative to an MRI device and ultrasound transducer can be compensated for when the motion is periodic (e.g., motion caused by a physiological cycle, such as a cardiac or respiratory cycle). See Denis de Senneville, B., Mougenot, C., and Moonen, C., Real-Time Adaptive Methods for Treatment of Mobile Organs by MRI-Controlled High-Intensity Focused Ultrasound, Magnetic Resonance in Medicine 57:319-330 (2007). In particular, an MRI of a target tissue mass currently acquired during a treatment procedure can be compared to reference MRI images of the target tissue mass previously acquired during a periodic cycle before the treatment procedure, which information can then be used to predict the displacement of the target tissue mass relative to the MRI device and ultrasound transducer. The ultrasound beam generated by the transducer can then be electronically controlled in real-time to continually maintain its focal zone at the moving target tissue mass. While this technique may be successful when tracking a target tissue mass that moves in accordance with a periodic cycle, it does not address non-periodic movement of the target tissue mass; for example, when the patient moves within the imaging device.
There, thus, remains a need for an improved method and system that can correlate relative displacement between an imaging device, a therapeutic device, and a target tissue mass in real time.